Aluminum oxide films



United States Patent Office it 2, ,037 Patented July 2, 1957 ALUMINUMOXIDE FILMS Preston Robinson, Williamstown, Mass., assignor to SpragueElectric Company, North Adams, Mass, a corporation of Massachusetts NoDrawing. Application May 13, 1953, Serial No. 354,873

3 Claims. (Cl. 204-42) The present invention relates to a new andimproved type of thick, relatively non-porous aluminum oxide film. Foryears, aluminum oxide films have been used for a Variety of purposes inthe electronics arts; Perhaps the most important of these uses has beenas an insulator between the electrodes of an electrolytic capacitor.Here such films have been extremely advantageous because of their densecharacter and their high dielectric constant. However, even in this use,it has been desired to obtain films possessing the basic characteristicsof aluminum oxide, but which are thicker than the presently used oxidefilms which do not exceed 1 mil in thickness. With such thicker films,higher voltage applications are made possible.

The object of the present invention is to produce a new and improvedvariety of aluminum oxide film which is characterized by being thickerthan, but similar to, the prior art aluminum oxide dielectric films.Further objects of the invention, as well as the advantages of it, willbe apparent from the balance of this specification, as well as theappended claims.

Briefly, the above aims are achieved by first oxidizing the surface ofaluminum electrode in an ionogen, such as oxalic acid, sulfuric acid,phosphoric acid, or the like so as to produce a comparatively porouscolumnar inert film, then depositing aluminum metal within the pores ofthe so-produced film, and finally oxidizing the sodeposited metal in anionogen, such as boric acid, which is capable of forming an extremelydense adherent oxide layer which effectively fills the pores of thefirst produced oxide.

The extreme simplicity of the present invention makes it extremelydifficult to describe in the usual detailed manner. The first oxidationtreatment is carried out by utilizing usual anodic oxidation proceduresand electrolytes, such as oxalic, sulfuric, phosphoric, or the like,which are known to produce comparatively porous oxide films of columnarstructure. In general, voltages up to about 400 volts and currentdensities of from 5 to about 40 amps. per square centimeter cansatisfactorily be employed during this step. The temperature of theelectrolyte is a comparatively immaterial factor, although it is ingeneral preferred to use temperatures of from about 20 C. up to aboutthe boiling point of the electrolyte.

The deposition of aluminum within the pores of the oxide film producedas set forth in the preceding paragraph is slightly more difficult thanthe formation of the initial oxide layer. To accomplish the deposition,the oxide layer is preferably immersed within a plating bath, atapproximately room temperature, such as are commonly employed in thedeposition of aluminum, and is connected as a cathode in such a bath. Asuitable electrolytic composition comprises the dispersion of toluene ina toluene solution of a fusion product of ethyl pyridiniumbromide andaluminum chloride with or without additional agents such asmethyl-t-butyl ether. More specifically, this plating bath has acomposition of 32% by weight of the fusion product of one mol of ethylpyrldlniumbromide and two mols of aluminum chloride,

67% of toluene and 1% of methyl-t-butyl ether. Other plating baths arewell-known to the art and are preferably baths wherein the electrolyteis dissolved in an organic solvent such as pyridine in which electricaldeposition can take place at room temperatures. The precise voltages andcurrents used throughout such deposition are well-known to the art. Ingeneral, the deposition of the aluminum should be carried out just untilthe point at which this metal is visible on the surface of the oxidefilm being treated, when viewed through a comparatively low powermicroscope, for example, a common microscope of 200 power. Furtherdeposition of the aluminum is apt to destroy the advantageous resultsobtained by, in effect, creating a second layer of aluminum on top ofthe initial base oxide layer; the second layer of aluminum then beingconverted to an oxide film of normal thickness. For certainapplications, a floating electrode structure of this type may bedesirable, but such ramifications are outside the broad scope of thepresent invention.

Following the aluminum deposition procedure specified in the precedingparagraph, a final oxide film is produced by immersing the coatedaluminum body in an electrolyte,

such as an aqueous solution of boric acid, in which anextremely densenon-porous oxide film can be obtained in a manner known to the art forthe same periods used presently in the formation of oxide films. Ingeneral, in the formation of this second oxide film, voltages up toabout 600 Volts are satisfactory, utilizing an initial current densityof 40 amps. per square centimeter, when the electrolyte consists of asaturated solution of boric acid held at its boiling point.

It is contemplated that the broad teachings of the present inventiveconcept will be extended to other valve metals, such as, for example,tantalum, zirconium, and the like, and to extremely thick adherentdielectric films to be produced on them by the expedient of firstdepositing a comparatively porous oxide film, then filling the pores ofthis initial film with a metal, such as the base metal, and thenoxidizing the final deposited metal so as to produce an extremely denseadherent film. It is to be understood that within the confines of thisinvention, it is substantially immaterial whether the second metal usedto fill the pores of the first oxide layer is the same or another valvemetal as the initial base material.

It is further contemplated that the broad teachings of the invention asdeveloped by further experimentation will show that extremelyadvantageous aluminum oxide and other valve metal oxide films can beobtained by first oxidizing these metals to produce a porous oxidelayer, then orienting the initial oxide films, as by the use of electricfield of reasonably high intensity, and then finally forming anextremely dense oxide layer as described above within the pores of theso-oriented initial porous oxide layer without the use of a second metalto fill the pores of the first layer.

As many apparently widely different embodiments of my invention may bemade without departing from the spirit and scope hereof, it is to beunderstood that my invention is not limited to the specific embodimentshereof except as defined in the appended claims.

What is claimed is:

l. The process of forming an insulating layer of oxide upon aluminumcomprising the steps of anodically forming a porous layer of oxide uponthe aluminum, electrodepositing a coating of aluminum within the poresof the oxide layer, and converting said deposit into the oxide.

2. The process of forming a heavy insulating layer of oxides upon analuminum comprising the steps of anodically oxidizing the aluminum at avoltage of up to about 400 volts and a current density of from about 5to about 40 amperes per square centimeter, plating a coating ofReferences Cited in the file of this patent UNITED STATES PATENTS WorkJuly 10, 1934 Fischer Apr. 7, 1936 Lilienfeld Apr. 13, 1937 Rankin eta1. June 6, 1939 Cohn Dec. 11, 1951

1. THE PROCESS OF FORMING AN INSULATING LAYER OF OXIDE UPON ALUMINUMCOMPRISING THE STEPS OF ANODICALLY FORMING A POROUS LAYER OF OXIDE UPONTHE ALUMINUM, ELECTRODEPOSITING A COATING OF ALUMINUM WITHIN THE PORESOF THE OXIDE LAYER, AND CONVERTING SAID DEPOSIT INTO THE OXIDE.